The use of components fabricated from fiber reinforced synthetic resins is receiving widespread acceptance in the aircraft, automotive, leisure products and industrial equipment industries. The selected use of such fiber reinforced synthetic resin components provides for significant weight reduction without sacrificing physical strength properties and durability. In the automotive industry in particular, attempts to improve fuel economy of automobiles and trucks has occasioned a replacement of various panels and structural members formerly comprised of steel with light-weight, high-strength components comprised of fiber reinforced plastics. Weight savings in many instances of as high as 70% are attainable without a sacrifice in durability, strength and operating performance.
It has been common practice to form such panels and structural components from sheet molding compounds (SMC) comprising a fiber mat or tape pre-impregnated with a partially cured flowable thermosetting resin which subsequently is compression molded with heat to effect a curing of the resin producing a finished article of the desired configuration. Thermosetting resins employed in the formation of sheet molding compounds or so-called "prepregs" include polyester resins, epoxy resins and polyimide resins of which polyester-type resins are most common. The fiber reinforcement conventionally comprises chopped glass fibers or continuous glass filaments which may be randomly oriented or directionally oriented to provide optimum strength properties of the final molded component. In many instances, such glass fiber reinforced plastics have exhibited inadequate strength in satisfactorily replacing steel counterparts or have necessitated the use of relatively thick sections to provide satisfactory strength resulting in an increased cost of the component and, in some instances, resulting in a physical size which cannot be accommodated in the space available.
In order to increase the strength of such fiberglass reinforced plastic components, it has heretofore been proposed to mix higher strength fibers or filaments with the glass fibers or glass filaments effecting a further reinforcement thereof. Fibers such as carbonized fibers, boron fibers, steel fibers, asbestos fibers, and the like, have been suggested of which highly carbonized or graphitized fibers are particularly suitable because of their exceedingly high-strength. Unfortunately, the use of such high-strength fibers or filaments such as graphite fibers alone or in combination with glass fibers substantially increases the cost of the fiber reinforced component detracting from a more widespread adoption thereof because of economic considerations. During the compression molding of such sheet molding compounds, trimming operations before and after the molding operation produce scrap which cannot be readily recycled further detracting from the economy of the operation.
The present invention overcomes the problems and disadvantages associated with prior art fiber reinforced plastics and methods of forming structural components therefrom in which the high cost high-strength fibers are strategically employed only in localized areas of anticipated high stress maximizing their contribution to high-strength and a corresponding reduction in weight and cost of the final product.